Changing of snowmelt, runoff timing threatens fish

Snake River in Summit County, Colorado.
Earlier snowmelt and runoff in Colorado streams could mean big trouble for fish. PHOTO BY BOB BERWYN.

Changes in runoff timing have been studied for impacts to reservoir operations and diversions, but what about aquatic and riparian ecoystems?

By Bob Berwyn

SUMMIT COUNTY — Earlier snowmelt and runoff in Colorado have been well-documented over the past few years and the finding were reinforced once again in a press release from the U.S. Geological Survey last week.

Water managers are already adjusting reservoir and diversion operations to account for the changes, but there’s been little discussion of the potential impacts to fish and other species that have evolved in tandem with historic streamflow regimes.

After another extensive study of snowmelt and peak runoff dates, the federal agency confirmed that peak runoff is coming as much as two to three weeks earlier than it did as recently as the 1970s — an astounding change in a short time, measured on the scale of Earth history.

The researchers also pinned the timing of snowmelt and runoff to changes in global and regional temperatures, as well as reduced snowfall during the study period. The published their findings in the Journal of Climate last week.

Water managers have already been scrambling to understand how the changes will affect operation of reservoirs and diversions for agricultural and municipal use, but the shift in timing could also have huge impacts on aquatic ecosystems in the southern Rockies and desert Southwest.

At issue is the growing gap between spring runoff flows and monsoon rains later in the summer. Fish native to the mountain streams of the region already live in a narrow window of flows and temperatures. If spring streamflows drop earlier in the year, trout and other fish could be exposed to longer dry periods.

Many species may not survive, said Shaula Hedwall, a biologist with the U.S. Fish and Wildlife Service, the federal agency charged with ensuring survival of endangered species. In many streams in the region, habitat has already been impacted by diversions, Instead of continuous streams, many have been chopped into segments of disconnected pools, severing the connection between populations.

“I expect that it will even further limit the amount of habitat,” she said. Hedwall and her colleagues have had to undertake intensive management efforts to maintain populations of some aquatic species.

The changes could also result in fish spawning earlier. Biologists think that fish may be able to adapt to those changes, but the real issue is year-round habitat. With longer, drier summers, it’s likely that many young fish won’t have enough habitat to survive.

Water stored in reservoirs could provide a buffer against shrinking aquatic habitat if it’s used for environmental purposes. But if the overall water supply shrinks while demand grows, it will mean making some serious choices down the road.


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